Update FreeBSD 12 CI image to 12.3.  12.2 is EoL.
Update FreeBSD 12 CI image to 12.3.  12.2 is EoL.

11.4 is also EoL, but we need to keep testing on FreeBSD 11 since we
expose a FreeBSD 11 ABI.

rust-lang/libc,rust-lang/libc,rust-lang/libc